The molecular cloning and the expression of the Salmonella typhi 36 kDal porin in Escherichia coli, the sequence analysis of its gene, the preparation of monoclonal antibodies against this antigen, and immunoblots and radioimmunoprecipitations assays of labeled ST whole cells have indicated that epitopes of this porin protein are immunogenic and exposed on the surface of ST. The same studies indicate that some of the exposed epitopes correspond to regions of the porin that are available among different species of Enterobacteria and that the immune response to them could be used to detect infection by S. typhi. Thus, purified 36 kDal S. typhi porin will be used to develop assays to detect antibodies against the porin, in the sera of typhoid fever patients, using an ELISA assay. The specificity and sensitivity of this antibody detection assay will be determined, compared to present diagnostic tests such as the Widal reaction, and then validated using a limited sample of well characterized sera from patients, and sera from patients with other bacteremic diseases and normal sera as controls. The need for this assay is clear, in light of the recrudescence of typhoid fever in developing countries, and the lack of adequate methodology to make an early diagnosis of the disease.